Oneworld RTW online tool planner is broken!
 

The online tool allows you to choose sectors in which no flights are available no matter which date you choose. For example: Athens-Istanbul, this segment doesn't seem to be supported on any oneworld carrier but the tool allowed me to build the itinerary and then balked only when I went to choose the specific flight :td:

Maybe it allowed you to build it into your itinerary originally because technically there is OW service between the two cities, with a stop (ie MAD, BUD, AMM, LHR) but then when you looked for flights either you selected 'direct flights only' or there was an error and connecting flights weren't displayed properly? Just a guess...

Simply search each leg separately. Likely there are no "nice" connections that exceed minimum connection times and not ridiculously long. (Or if use "plan and book" option then there could be flights but with no availability - this can easily be tested by using the "plan" option instead.)

There definitely is an Athens -Instanbul segment supported by Oneworld carriers, its just not a direct flight. I flew this segement ( in reverse) on a RTW award ticket last year. Using Malev Air - Istanbul - Budapest - Athens or alterntively another option is with Iberia - Istanbul -Madrid-Athens. Unfortunately the connection times are crap - if i remember correctly, I arrived from Istanbul at 6:30am and had to wait approx 5 hours for my connecting flight to Athens. ( This was May last year, so the timetable has probably changed)

I agree with Kiwi Flyer, you need to search the segements separately.